Splitting Water to Get Hydrogen!

THIS, I dare say, is the holy grail for those trying to extract energy from H2 without creating CO2 greenhouse gas. Fuel cells would thrive if a cheap, efficient and renewable source of H2 was to become possible. And what is THIS? By THIS I mean the science of splitting water into its components H2 and O2 at close to room temperature using free energy such as sunlight.

Lately I have seen several interesting technologies that are helping move the ball closer to that goal. I can’t highlight all of them here – some I am under a confidentiality agreement on – but things do look good, and I am hopeful that a fundamental (and major) breakthrough may be just around the corner, and the field may be ripe now for entrepreneurship and commercialization to take hold.

I will use this post to highlight one such discovery that was reported in the Journal of American Chemical Society. I thank Green Car Congress, and Mike Milliken, for highlighting it on their blog.

Michael Grätzel and his colleagues have developed a device that sets a new benchmark for efficiency in splitting water into hydrogen and oxygen using ordinary sunlight. The research will be published in the 13 December issue of the Journal of the American Chemical Society.

Previously, the best water photooxidation technology had an external quantum efficiency of about 37%. The new technology’s efficiency is 42%, which the researchers term “unprecedented.” The efficiency is due to an improved positive electrode and other innovations in the water-splitting device.

Grätzel and collaborators developed the Grätzel Cell, a dye-sensitized photoelectrochemical cell that uses photo-sensitization of wide-band-gap mesoporous oxide semiconductors. The work originally appeared in a paper in Nature in 1991.

As reported in the Nature article, the original overall light-to-electric energy conversion yield of a Grätzel cell was 7.1–7.9% in simulated solar light and 12% in diffuse daylight.

Iron oxide (-Fe2O3, or hematite) is an especially attractive photoanode due to its abundance, stability, and environmental compatibility, as well as suitable band gap and valence band edge position. Unfortunately, the reported efficiencies of water oxidation at illuminated hematite electrodes are notoriously low.

Grätzel and his team tackled that in this most recent work by producing Fe2O3 photoanodes via deposition of silicon-doped nanocrystalline hematite films by APCVD (atmospheric pressure chemical vapor deposition).

The result was a highly developed dendritic nanostructure of 500 nm thickness having a feature size of only 10-20 nm at the surface. The dendritic nanostructure minimizes the distance photogenerated holes have to diffuse to the Fe2O3/electrolyte interface in a film that is thick enough for strong light absorption.

The efficiency is further enhanced by deposition of a thin insulating SiO2 layer below and a cobalt monolayer on top of the Fe2O3 film.

Under illumination in 1 M NaOH, water is oxidized at the Fe2O3 electrode with higher efficiency (IPCE [incident photon to current efficiencies] = 42% at 370 nm and 2.2 mA/cm2 in AM 1.5 G sunlight of 1000 W/m2 at 1.23 VRHE) than at the best reported single crystalline Fe2O3 electrodes.